The present invention relates to a silicone-based antifoam composition or, more particularly, to a silicone-based antifoam composition readily emulsifiable with good dispersibility in water or dispersible in organic solvents and capable of durably maintaining the antifoam activity without decrease over a long period of time even under vigorous agitation or under an alkaline condition as well as a method for the preparation thereof.
Among various types of antifoam compositions, as is well known, silicone-based antifoam compositions comprising a dimethyl polysiloxane as the principal ingredient are widely used, by virtue of their excellent properties or performance in comparison with those of other types, as an agent for breaking or suppressing rising foams in a variety of industrial fields including chemical industry, food industry, petroleum industry, textile industry and the like. A problem in the use of a silicone-based antifoam composition in general is the relatively poor durability of the antifoam activity and, in particular, the antifoam performance thereof is rapidly lost under an alkaline condition or under vigorous agitation as is the case in the jet dyeing of textiles and in the aeration tank for sewage disposal.
Accordingly, various attempts and proposals have been made including a method described in Japanese Patent Publication No. 52-31836, in which the finely divided silica powder formulated in conventional silicone-based antifoam compositions is rendered hydrophobic in advance by the treatment with dimethyl dichlorosilane and the like, and a method described in Japanese Patent Publication No. 51-35556, in which the silica powder is treated with a nitrogen-containing organosilicon compound. These methods of the hydrophobic treatment of the silica powder, however, are economically not advantageous due to the long time taken for the treatment and the necessity of installing special facilities therefor. Alternatively, a silicone-based antifoam composition is proposed in Japanese Patent Kokai No. 57-48307, which is prepared by admixing an organopolysiloxane with an organohydrogenpolysiloxane, a silica powder and a catalyst. The antifoam compositions of this type are disadvantageous due to the danger of possible fire and explosion since hydrogen gas is evolved from the composition during use in addition to the not always satisfactory durability of the antifoam activity thereof.
It is also known that the durability of the antifoam activity of a silicone-based antifoam composition can be improved when the composition is prepared by using a high-molecular silicone fluid having an increased viscosity. The disadvantage in the compositions of this type is the difficulty in handling and emulsification thereof due to their high consistency. Although such a composition can be used in some applications in the form of a dispersion in an organic solvent such as hydrocarbons, a danger of fire is unavoidable in such a solvent-containing composition and the field of applicability thereof is limited to non-aqueous foaming liquids.